As I understand this, this is an easy but still explicit concurrency construction, which is not what haxl does. It's easy to imagine how something like the toy example of friend-list-intersection could be converted to explicit concurrency in languages with good concurrency support. This is an easy optimization to make by hand in the cases where concurrency is obvious at some layer or within some abstraction. The power of haxl comes when the two (or whatever) requests come from wildly different places in the AST. For example, if you combine the result of friend-list-intersection with some other numeric quantity that's the result of some other fetches. Haxl essentially performs this optimization automatically, over the entire program.

Something like...

    renderPage :: Haxl Html
    renderPage = renderHeader + renderBody + renderRightPane + renderFooter

This will travel as far as possible through all paths in the AST collecting all IO to be performed in the first round (which, once fetched, will unblock more of the AST, and the process repeats until we have an answer).